Why do lab reference ranges differ between laboratories?

Reference ranges are not universal constants. They are established by each laboratory based on the equipment and methods it uses and the population it tests, which is why the same value can sit inside the range at one lab and just outside it at another.

This matters most when you are comparing results over time. A change between two tests done at different laboratories may reflect the method rather than your body, which is one reason clinicians prefer to track trends within a single lab where possible.

It is also why a result marked outside the range is a prompt for interpretation rather than a verdict. MedlinePlus is explicit that results are read alongside your symptoms, history and other tests, which is work for the clinician who ordered them.